As the dimensions of semiconductor devices and components continue to decrease, the demand for semiconductor wafer and photomask inspection systems exhibiting high throughput and improvements in resolution continue to increase. One manner in which higher levels of resolution are attained in semiconductor and photomask inspections systems includes the utilization of an illumination source capable of emitting shorter wavelength light.
Certain practical advantages may be achieved when illuminating a wafer or reticle with light having wavelengths at or below 400 nm. Providing suitable lasers for high quality wafer and photomask inspection systems presents a particular challenge. Conventional lasers capable of generating deep ultraviolet (DUV) light energy are typically large, expensive, and suffer from relatively short lifetimes and low average power output. In order to obtain adequate throughput and defect signal-to-noise ratio (SNR), wafer and photomask inspection systems generally require a laser based illumination source having high average power, low peak power, and relatively short.
Conventionally, the primary method for providing adequate DUV power entails converting long wavelength light to shorter wavelength light, referred to herein as “frequency conversion.” It is well known in the art that frequency conversion from longer wavelength light to shorter wavelength is often accomplished utilizing one or more non-linear optical crystals. In this context, frequency conversion requires high peak power light in order to produce a nonlinear response in a given non-linear optical crystal. To increase the efficiency of this process the longer wavelength light may be generated to have high average power, short optical pulses, and may be focused into the optical crystal. The original light “longer wavelength” is commonly referred to as “fundamental light.”
Generating light at wavelengths below 400 nm, and especially below 300 nm, is challenging. Light sources implemented in semiconductor inspection systems require relatively high powers, long lifetimes, and stable performance. Light sources meeting these requirements for advanced inspection techniques are nonexistent in the prior art. The lifetime, power, and stability of current DUV frequency converted lasers are generally limited by the implemented frequency conversion crystal and frequency conversion scheme. This is particularly true for non-linear conversion crystals exposed to DUV wavelengths, such as, but not limited to, 355, 266, 213, and 193 nm.
Many inspection applications require the frequency converted laser power or wavefront to remain stable over time. Due to degradation of the nonlinear optical crystal, as a result of exposure to the illumination, maintaining power and wavefront stability over time is challenging. In order to extend the lifetime of frequency conversion crystals, it is common to shift a given crystal such that an impinging laser beam focuses on an unused portion of the crystal prior to the degradation of a current location beyond acceptable limits. In another aspect, the optical crystal may be continuously shifted at a rate that prevents the onset of wavefront or power damage.
Lifetime of frequency conversion crystal sites, however, may vary significantly from crystal site to crystal site. The current method for dealing with variance in crystal site lifetime includes choosing a crystal site lifetime based on the shortest expected lifetime, with some safety margin included. This method may dramatically limit the lifetime of a given optical crystal. In addition, this method may suffer from incorrect estimations, as one or more crystal site locations may degrade faster than anticipated. This may adversely impact the performance of high precision equipment using this type of a laser as a light source
Accordingly, it may be desirable to provide a method and/or system which provide a frequency conversion system equipped with crystal site lifetime monitoring capabilities.